Dependent Dropdown Validation Lists

I am working on a order form which I need various dropdowns dependent on one another. the one I am having trouble with is I need the Jamb Type list to equal the jamb size list depending on which jamb type is picked.
3/4" Thick Standard Beveled jamb = 4-9/16", 6-9/16"
1" Thick Sheet rock return flat jamb = 6", 8"
1-1/4" rabbit jamb = 6", 8"
I am having trouble getting this to happen. I had done it with a nested IF function for previous dropdowns but this time it keeps returning and error and I cant figure it out.